A general vapor cycle system is basically an air conditioning system, i.e. it has the same essential elements as the general components of a home or car air conditioning system such as a compressor, a condenser and an evaporator. In most household and even most car applications, there is typically a single evaporator.
U.S. Pat. No. 4,539,823, illustrates a typical known refrigeration system which provided a single evaporator control. This system attempted to avoid unnecessary power consumption due to the lack of refrigerant in the evaporator and to eliminate variations in the air temperature immediately downstream of the evaporator. To accomplish this, the system used compressor capacity to control air temperature. However, this arrangement was intended for a single evaporator. It used a reciprocating compressor with variable displacement. A thermal expansion valve (TXV) appears to have been mechanically/pneumatically driven. The TXV controlled superheat at the evaporator exit.
U.S. Pat. No. 4,633,675, involves a system for controlling compressor capacity which was also limited to a single evaporator system in which there is provided a reciprocating compressor with variable displacement driven by a car engine in accordance with the load of the A/C system. Again, this device uses compressor capacity to control heat load via measurement of the air temperatures across the evaporator.
In a typical closed-circuit vapor cycle system, a refrigerant fluid is compressed and supplied to an upper heat exchanger which is a vapor condenser (with some superheat). The condenser exchanges heat outwardly through, for example, a counterflow of source cooling water or air. The cooled refrigerant fluid is then expanded through a thermal expansion valve to reduce the pressure and flash the refrigerant into a partially gaseous state and is then passed through an evaporator to exchange heat inwardly at which point the refrigerant is in the vapor state. The vapor is then elevated by a compressor to a higher pressure and condensing temperature so that it will liquify in its transfer of heat to atmospheric level. In order to prevent damage to the compressor, it is desired that the refrigerant fluid admitted to the compressor be a superheated, nearly dry saturated vapor. It was known that second lower temperature evaporator could be added but this created problems in that the refrigerant from the evaporators mixes together.
The basic concept is to cycle refrigerant through a condenser or out of a condenser with cold refrigerant emerging from the condenser. The cold refrigerant is then expanded across a thermal expansion valve where the refrigerant pressure drops so that the refrigerant in effect flashes up into a two-phase liquid/gas mixture. The evaporator in this type of VCS has counterflow or source fluid which can be, for example, air returning on the other side of the evaporator.
However, the vapor cycle system under consideration is more specifically directed to the type of unit used in aircraft, particularly large transport aircraft. In aircraft, unlike household and automotive applications, there is often a desire to incorporate more than one evaporator in a single circuit even though it is only necessary to dump all the heat generated in the aircraft into one heat sink which can either be the stored fuel or outside ambient air. In such VCS's, variable speed compressors have been used with their speed modulated based upon evaporator load temperature and their thermal expansion valve position modulated based upon evaporator exit refrigerant superheat. With multiple evaporators in one circuit, there can be, for example, an evaporator in the cockpit cooling off the cockpit heat load and one in the main cabin cooling off the main cabin load. The temperatures in the cockpit and in the main cabin can be controlled independently while all of the heat load from the main cabin and the cockpit can be dumped to a single heat sink such as the ambient air. However, when the VCS includes multiple evaporators, the compressor speed cannot independently control multiple load temperatures where, for instance, one evaporator is cooling off the cockpit load and another evaporator is cooling off a main cabin load.
Prior to the present invention, it was further known that a single compressor and a common condenser could be used with an invertor circuit control for a variable speed compressor. In that arrangement, however, the inverter circuit control section received signals from a pressure sensor which detected the suction side of the compressor. While this arrangement had the advantage of a stepless frequency control, it suffered from slow response and difficulty in detecting slight pressure changes.
The prior art was also cognizant of the importance of protecting the compressor by keeping the refrigerant in the superheat range when introduced into the compressor inlet. However, the importance of controlling temperature at the exit of an evaporator to obtain higher evaporator efficiency while maintaining sufficient superheat was not recognized or achieved.
Systems using multiple evaporators were also known as previously mentioned. U.S. Pat. No. 4,658,596, discloses a refrigerating apparatus with a single compressor and multiple evaporators in which an additional set of electromagnetic valves were used, one for each evaporator. This apparatus aimed at providing more accurate compressor drive control by detecting the temperature of the refrigerant flowing through a bypass at the outlet of the compressor such that when one evaporator was closed the bypass temperature fell and the compressor operating frequency was adjusted. Because of the change in the operating frequency, the saturated or vapor temperature of the evaporators in correspondence to the pressure of the low pressure side was kept at a constant value. An electronic control circuit compared a detected temperature signal at a bypass pipe between a tank and the refrigerant side of the accumulator with a prescribed temperature. If the detected temperature was lower than the prescribed temperature, the control circuit sent a control signal corresponding to the temperature difference to the inverter circuit. The electromagnetic valves were of the on/off type, and the TXVs might have been pneumatically controlled. Compressor speed was controlled by the sink temperature, and the TXVs controlled evaporator exit superheat while the electromagnetic on/off valves controlled evaporator temperature. Consequently, each evaporator required a separate electromagnetic valve to control evaporator temperature and a thermal expansion valve to control exit superheat.
Microcomputer-based refrigeration systems utilizing a variable speed compressor and a motorized expansion valve are known as shown, for example, in a paper entitled "Digital Control System for a Refrigerator Heat Pump for Spacecraft Environment" by D. Parnitzki (describing what is hereinafter referred to as "the Parnitzki system") presented at the 18th Intersociety Conference on Environmental Systems in San Francisco, CA in July 1988. The Parnitzki system was a heat pump system which transferred power from a heat generating payload to an evaporator wherein refrigerant was evaporated after leaving the compressor in a vapor state at an elevated temperature and pressure, and was then converted to liquid within an accumulator-condenser where an outward heat exchange was generated. The liquid refrigerant then passed through a narrow opening in the expansion valve and left the valve at low temperature and pressure. A heat exchanger was provided between the accumulatorcondenser and the expansion valve in the system circuit to subcool the refrigerant before entering the expansion valve in order to reduce vapor bubbles which could otherwise clog the valve and to make the refrigerant slightly overheated vapor when it left the evaporator to increase the efficiency of the compressor.
However, the Parnitzki system was designed for a spacecraft environment where the compressor must accept incompressible liquid at its input without any damage, as may occur at zero gravity. The motorized expansion valve had a mechanical pressure control loop in which a spring loaded diaphragm controlled the valve opening so that the valve opening kept the output pressure constant roughly proportional to the spring force. Thermistors were provided for control and monitoring purposes as were pressure sensors.
The Parnitzki system also proposed, inter alia, a vapor overheat regulator scheme to control compressor speed, but this scheme proved to be unsatisfactory. In particular, a constant reference overheat temperature, Tohref, had the actual degree of vapor overheat, Toh, subtracted therefrom, and the resulting deviation from the desired degree of overheat, Toherror, was supplied to a regulator for the compressor motor speed. Although this scheme was described as having been found useful for regulating small deviations from the optimum operating states, it was not deemed satisfactory for large deviations. This is due to the physical characteristics of heat exchangers where, on one hand, if the refrigerant flow rate is increased in the evaporator at a constant pressure the amount of superheat will decrease and, on the other hand, if the refrigerant flow rate is constant and the pressure is decreased, the superheat will increase. Instead, a regulator scheme was adopted which regulated the temperature difference between evaporator wall and refrigerant to a constant value by way of a pressure loop and thereby the temperature of the refrigerant entering the evaporator was controlled.
The Parnitzki system was also based on a mechanical pressure control motorized expansion valve which incorporated internal pressure feedback. Rather than control the flow area by changing the position of a poppet valve, the Parnitzki system valve changed compression on a preloaded spring which acted on a diaphragm to change the operating pressure of the evaporator. The diaphragm had the pressure of the evaporator refrigerant, and this pressure acted on the diaphragm which was spring loaded. As the spring loading was changed, the flow area changed and thereby the pressure which balanced out against the spring force. The compressor controlled evaporator refrigerant flow, and the thermal expansion valve controlled pressure. If the degree of vapor overheat was too high, then the vapor overheat regulator reduced it by increasing the compressor speed. In other words, increased compressor speed reduced the overheat. If it is assumed there was a constant valve position, increases in compressor speed produced more flow because pressure begins to decrease in the evaporator and open the valve further with the net effect of increasing the refrigerant flow rate in the evaporator while the pressure remains constant with a decrease in superheat. In any event, multiple evaporators could not be controlled with this arrangement because the thermal expansion valve controlled pressure, and the pressures were common.
Although it was known prior to the present invention that it was important to protect a compressor by keeping the refrigerant being fed thereto in the superheat range, it was not recognized that it was also important when using multiple evaporators to control the air temperature at the exit of the evaporators.
Although the prior art show various systems and techniques to control compressors associated with evaporators, the conventional systems did not use the concept of varying the compressor speed as a function of pressure at the inlet of the compressor and evaporator and temperature.